This invention relates generally to the joining of articles by a fusible filler material, and more particularly, to an economically manufactured resistance element which may be placed in a cavity between two articles to be joined and then fused to form a bonding medium by heat generated on the passage of an electrical current through the resistance element.
Thermoplastics are widely used construction materials for a variety of articles, including pipes and fittings, due to their corrosion resistance, light weight, ease of handling, fabricability and relatively low cost. A variety of joining techniques have been developed to broaden the range of usefulness of thermoplastic articles. Mechanical joining techniques such as threaded joints and flange connections require specialized parts and sometimes laborious assembly, and in many cases may not be sufficiently fluid tight for pressurized piping systems. In chemical joining techniques such as solvent welding, a solvent is applied to the surface of the parts to be joined, and the parts are placed in contact before the dissolved portion solidifies, thereby producing a sound, water tight joint. However, chemical joints are sometimes not feasible because the articles must be placed in contact immediately after the application of the solvent. Additionally, some types of thermoplastic materials are highly resistant to conventional, safely usable solvents.
The thermal bonding technique for joining thermoplastic materials utilizes heat to fuse a fusible filler material and preferably a portion of the articles to be joined themselves, so that after cooling the articles are joined by a sound, fluid tight, sealed joint. In a typical application of thermal bonding to join a water pipe and a fitting, an annular recess is provided in the fitting simultaneously adjacent the inner wall of the fitting and the outer wall of the pipe to be joined. A heated coil comprising an electrical conductor covered by a fusible thermoplastic material is inserted into the recess, the pipe is placed within the heating coil so that the heating coil is positioned between the inner wall of the fitting and the outer wall of the pipe, and an electrical current is passed through the electrical conductor to heat and fuse the thermoplastic material and adjacent portions of the walls of the fitting and the pipe. During cooling, a bond forms between the fitting and the pipe.
The primary requirements for a heating element are the provision of an electrically conductive path in a wire which will be heated with the passage of a current, a geometry which makes the wire leads externally accessible, the introduction of a thermoplastic filler material into the region of the bond, and, preferably, a structure which allows the heating element to be readily and economically fabricated. In one approach to a cylindrical heating element, a wire is doubled and wound pairwise onto a thermoplastic core and then covered with a thermoplastic coating, but this heating element is cumbersome to produce in mass quantities and somewhat restricted in use. In another approach, a single conductive wire is covered with a thermoplastic coating serving both as insulation and as a filler in the joint, and wound into a spiral heating element. External accessibility of both wire leads is accomplished by crossing one end over the wound coil, but this design is inconvenient to fabricate and may result in an electrical short, and the crossover is found to be a potential point of leakage of fluid through the joint in pressurized systems. Yet other designs for heating elements have been tried, but none have met the requirements set forth above, and in particular these designs usually require costly hand layup or handling during fabrication.
Accordingly, there has been a need for an approach to fabricating heating elements for joining thermoplastic materials such as pipes and fittings, which is both convenient to use and economical. The present invention fulfills this need, and further provides related advantages.